This invention relates to a power steering control device. More particularly, the invention relates to a novel steering power control device which varies the steering power in response to the combined factors of the weight of a load that is carried on the vehicle and the speed of the vehicle.
Power steering gives an auxiliary force to the turning effort of a steering wheel and it facilitates the operation of the steering wheel. There are known several kinds of power steering systems such as an integral type, a semi-integral type and a linkage type. The linkage type power steering systems are further divided into two kinds, that is, a combined type power steering in which a power cylinder and a control valve are integrally formed and the other type is a separate type power steering system in which the above two elements are formed separately. In order to give the feeling of steering resistance to the driver, any one of these conventional power steering systems is inevitably provided with a reaction force mechanism which transmits a reaction force to a steering wheel in response to the intensity of a steering output. With respect to the manners to of applying reaction forces to steering wheels, the power steering systems are classified as follows: In a first type power steering system, the intensity of the reaction force to the steering wheel is determined in response to the pressure in a working chamber which generates the steering output, that is, in response to the ratio of auxiliary force of the steering power. In a second type power steering system, a reaction force chamber to give resistance to the operation of the steering wheel is formed in addition to the working chamber, thereby the reaction force applied to a steering wheel is determined by the level of the pressure in the reaction force chamber. The other third type power steering system is a combined type of the former two types, in which the reaction force applied to the steering wheel is generated in response to the ratio of an auxiliary force and to the reaction force of a reaction force chamber. In any of the above three type power steering systems, when the working pressure caused by the power steering is large, the resistance in the operation of a steering wheel becomes relatively large.
In recent years, in order to secure safety in high speed running, various kinds of pressure or flow control means have been proposed so as to enlarge the reaction force which is applied to the steering wheel with the increase of vehicle speed even under the same ratio of steering output (ratio of auxiliary force). In any of such improvements in power steering systems, however, the speed of a vehicle is detected so as to control the reaction force applied to the steering wheel in response to the vehicle speed and any factor other than the vehicle speed is not taken into consideration.
The weight of a load of a motor truck is largely varied between a loaded state and an unloaded state. In such a motor truck, when the elements are set so as to obtain a proper steering output and a reaction force of the steering wheel in a loaded state, the operation of the steering wheel in an unloaded state becomes too light and the stability in vehicle running is not maintained, especially in high speed running. To the contrary, if the elements are set to an unloaded state, the resistance in the operation of a steering wheel becomes too large in a loaded state.